Problem in rear of car

[wannarace928]

I have started to have an issue with one of my SVX's and I am puzzled as to what could cause it. It was running great until today, today whenever I let off of the gas it makes a loud thumping coming from the rear end. This happens when the brakes are NOT applied, and when the brakes are applied from going 65-70mph. Does not happen if braking at slower speeds. At first I thought it was the muffler making a funny noise, like a popping, but it sounds more like something is loose because I can feel it in the car also. When looking under the back end I do not see anything obviously loose. Could this be the rear diff acting up?